Mount Karthala is an active shield volcano 2,361 metres high, and it is most of the island of Grande Comore, in the Comoros. Its lava is young and porous enough that the rain goes straight down through it, so the island has not one river. What falls on the mountain comes out under the sea. The summit is a caldera three kilometres by four, still steaming, with young cinder cones on its ash floor. The Chahalé pit in the middle of it held a lake until 2005, when magma reached the water and steam blasts emptied the crater. It has not come back, and what you look down on now is bare grey rock.
The walk up is the island in cross-section. It starts among banana and vanilla, climbs through rainforest, and then the trees shrink and quit. Above them the path runs through waist-high heather and ferns rooted in loose ash, cloud dragging across it most of the day and volcanic grit squeaking under your boots. Hours earlier you were under coconut palms at sea level. The last stretch is done in the dark, because by mid-morning the cloud usually seals the summit for the rest of the day. Comorian hiking clubs are on the mountain most weekends, and going up with one turns a hard two days into a shared one, with porters who read the cloud and know where the trail disappears. Down at the coast the same mountain has paved long stretches of shore in black rock and grey shingle, where the swell breaks hard and the water goes deep blue a few strokes out. The forest belt you climbed through holds a small owl that lives on this mountain and no other, heard far more often than seen. The lava does not stop at the shoreline either. It keeps going down the drowned slope, and in caves 150 to 250 metres below the surface lives the coelacanth, the fish that was thought to have been extinct for 65 million years. It rests in the rock by day and drifts out along the face at night.